1. Field of the Invention
The present invention relates to a gas concentration sensor and, more particularly, to a method for controlling an oxygen concentration sensor for sensing the oxygen concentration in the exhaust gas of an on-vehicle internal combustion engine when the sensor is active by using the element resistance thereof.
2. Description of the Related Art
There have been demands recently for improved accuracy in air-fuel ratio control of motor vehicle engines. In response to these demands, a linear air-fuel ratio sensor, or oxygen concentration sensor, has been developed. The sensor linearly detects, over a wide range, the air-fuel ratio of an air-fuel mixture sucked into the internal combustion engine corresponding to the concentration of oxygen in the exhaust gas. In order to maintain detection precision in such an air-fuel sensor, maintaining the air-fuel sensor in an active state is important. In general, the air-fuel sensor is maintained in an active state by supplying a current to a heater equipped to the air-fuel sensor and heating an element of the air-fuel sensor.
During excitation of the heater, there is conventionally disclosed a technique for sensing the temperature of the sensor element and thereby performing feedback control of the element temperature so that the element temperature reaches a desired activation temperature (e.g. approximately 700° C.). In this case, in order to sense the instantaneous element temperature, a method of equipping a temperature sensor to the sensor element and drawing out the element temperature from the sensed result is known and is commercially practiced. However, in this method, the cost is increased due to the necessity of adding the temperature sensor. On this account, it has been proposed to detect the resistance of the sensor element based on a prescribed correspondence relationship between the element resistance and the element temperature. Thus, it is thereby possible to draw out the element temperature from the detected element resistance. It is to be noted that the detected result of the element resistance is used, for example, also for determining the degree of deterioration of the air-fuel sensor.
FIGS. 32A and 32B are waveform diagrams illustrating conventionally used technique for detection of the element resistance. These Figures illustrate a case where a critical current type oxygen concentration sensor is used as the air-fuel ratio sensor for use in an internal combustion engine. Namely, before a point in time toll in FIGS. 32A and 32B, a prescribed voltage (a positive applied voltage Vpos) for the detection of the air-fuel ratio is applied to the sensor element. The air-fuel (A/F) ratio is determined from a sensor current Ipos output in correspondence with this applied voltage Vpos. Also, during a time period from t011 to t012, a negative applied voltage Vneg for the detection of the element resistance is applied, whereby a sensor current Ineg corresponding to this time period is sensed. By dividing the negative applied voltage Vneg by the corresponding sensor current Ineg, the element resistance ZDC is determined (ZDC=Vneg/Ineg). This detection procedure is generally known as a method of detection of the element resistance that uses the d.c. characteristic of the air-fuel ratio sensor.
The above-described conventional technique is one which detects element resistance (d.c. impedance) by applying a d.c. voltage to the sensor element. In contrast to this, Japanese Patent Laid-Open Publication No. Hei. 4-24657 discloses a technique of detecting element resistance by applying an a.c. voltage to the sensor element. The a.c. voltage is applied continuously to the air-fuel ratio sensor, and the resulting sensor output is passed through a low pass filter, and high pass filter, for separate air-fuel ratio calculations. Thereafter, the both air-fuel ratios are averaged to thereby determine the a.c. impedance. This procedure of detection is generally known as a method of detection of the element resistance that uses the a.c. characteristic of the air-fuel ratio sensor.
According to the above-described d.c. impedance method, the sensor current Ineg that is output when the negative rectangular wave applied voltage Vneg has been applied sharply fluctuates as illustrated in FIG. 32B. If the oxygen concentration is detected during this time period, it is impossible to detect a true oxygen concentration.
Also, according to the a.c. impedance method discussed above, since the air-fuel ratio is detected by passing the sensor output through the low pass filter, there arises the problem that a phase lag occurs in the air-fuel ratio output. Also, a.c. noises are liable to be superimposed on the air-fuel ratio output. These problems are prominent, particularly when the operational state of the internal combustion engine is in a transition state.
In an air-fuel ratio detection microcomputer, as the number of processings to be executed with the same timing increases, the processing load increases. The simultaneous detection processing of the air-fuel ratio, detection processing of the element resistance, and control processing of the element heater with respect to the oxygen concentration sensor all add to the processing load. As a result, processing time length exceeds the processing period, resulting in deviation of the timing of processing during subsequent period.
Further, because the sensor signal is small, when noises are superimposed thereon at the time of detecting the element resistance of the oxygen concentration sensor, the determined element resistance value differs greatly from a true element resistance value.
Further, when detecting the element resistance of the oxygen concentration sensor and thereby selecting the applied voltage from a relevant map, if noises are superimposed on the sensor signal, the selection made with respect to the map becomes unstable.